The bacteriology of acute salpingitis was studied in 87 patients. Gonococci were recovered from 18 patients. Lower yields of bacteria were isolated in the peritoneal fluid than from the vaginal cultures, and a poor correlation was observed between the peritoneal fluid and vaginal cultures. Anaerobic bacteria dominated in both sites. Staphylococci, streptococci, enterobacteria, peptostreptococci, peptococci, lactobacilli and bacteroides normally present in the vagina were the predominant isolates recovered from the culdocentesis. Paired acute and convalescent-phase sera obtained from patients with isolates of Bacteroides fragilis were assayed for antibody response to the polysaccharide antigen of B. fragilis. Significant titer increases were observed in sera from only two out of eight patients with non-gonococcal salpingitis and no titer changes were noticed in two patients with gonococcal. These findings suggest that B. fragilis only infrequently plays an etiologic role in acute salpingitis.